TRACE-ELEMENT AND U-PB ISOTOPE COMPOSITIONS OF PYRITE TYPES IN THE PROTEROZOIC BLACK REEF, TRANSVAAL SEQUENCE, SOUTH-AFRICA - IMPLICATIONS ON GENESIS AND AGE

The Proterozoic Black Reef is, in most cases, indistinguishable from t he gold-, uranium- and pyrite-bearing Archaean Witwatersrand reefs. Ho wever, the prevailing opinion that the Black Reef gold mineralization was largely derived from reworking of the underlying Kimberley Reef is not supported by the morphology, trace-element geochemistry and U-Pb isotope data on detrital pyrite in these reefs. The grade of metamorph ism recorded in the Black Reef Quartzite Formation is significantly lo wer than the 300-400 degrees C that has been proposed for the sediment s of the Witwatersrand Supergroup. The U-Pb isotope and trace-element geochemical signatures in pyrite have, therefore, been preserved in th e Black Reef. The morphological and compositional differences retained by the detrital and synsedimentary pyrite types, both occurring toget her in the Black Reef, imply that they were formed by different proces ses and in contrasting geochemical environments. In the Black Reef, th e data for the detrital pyrite are consistent with their derivation fr om a similar to 3000-2800-Ma granitic, probably hydrothermally altered , source region. By contrast, the geochemical and Pb-isotopic composit ion of the concretionary pyrite is interpreted to reflect its synsedim entary formation in a lagoonal or tidal flat environment prior to simi lar to 2500 Ma. The morphology, primary mineral inclusions and the unr adiogenic Pb-isotopic compositions preserved in the Black Reef detrita l pyrite are irreconcilable with models proposed for the formation of Witwatersrand pyrite by a process of pyritization of Fe-bearing detrit al minerals. Fluids present in the Black Reef depositional/diagenetic environment were anomalously enriched in radiogenic Pb and any transfo rmation process would not be able to escape acquiring this enriched Pb -isotopic signature.